U.S. patent application number 13/464370 was filed with the patent office on 2013-11-07 for multi-function beam delivery fibers and related system and method.
This patent application is currently assigned to Raytheon Company. The applicant listed for this patent is David A. Rockwell. Invention is credited to David A. Rockwell.
Application Number | 20130294728 13/464370 |
Document ID | / |
Family ID | 49512576 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130294728 |
Kind Code |
A1 |
Rockwell; David A. |
November 7, 2013 |
MULTI-FUNCTION BEAM DELIVERY FIBERS AND RELATED SYSTEM AND
METHOD
Abstract
An optical fiber includes multiple cores and a cladding. At
least one of the multiple cores forms an optical waveguide and has
an elongated cross-section with a narrower dimension in a fast-axis
direction and a wider dimension in a slow-axis direction. The
cladding surrounds the multiple cores and has a refractive index
that differs from at least one refractive index of the multiple
cores. The multiple cores could be stacked such that a first of the
multiple cores is located at least partially over a second of the
multiple cores in the fast-axis direction. The optical fiber could
include an additional core within the cladding and having a
substantially circular cross-section. The cores could be used to
transport a high-power laser beam, an illumination laser beam, and
an alignment laser beam. The optical fiber could have a length of
at least two meters.
Inventors: |
Rockwell; David A.; (Culver
City, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rockwell; David A. |
Culver City |
CA |
US |
|
|
Assignee: |
Raytheon Company
Waltham
MA
|
Family ID: |
49512576 |
Appl. No.: |
13/464370 |
Filed: |
May 4, 2012 |
Current U.S.
Class: |
385/39 ;
385/126 |
Current CPC
Class: |
G02B 6/4296 20130101;
G02B 6/02009 20130101; B23K 26/0604 20130101; G02B 6/02042
20130101; G02B 6/024 20130101 |
Class at
Publication: |
385/39 ;
385/126 |
International
Class: |
G02B 6/036 20060101
G02B006/036; G02B 6/26 20060101 G02B006/26 |
Claims
1. An optical fiber comprising: multiple cores, at least one of the
multiple cores forming an optical waveguide and having an elongated
cross-section with a narrower dimension in a fast-axis direction
and a wider dimension in a slow-axis direction; and a cladding
surrounding the multiple cores, the cladding having a refractive
index that differs from at least one refractive index of the
multiple cores.
2. The optical fiber of claim 1, wherein: the multiple cores
comprise first and second cores; and the multiple cores are stacked
such that the first core is located at least partially over the
second core in the fast-axis direction.
3. The optical fiber of claim 1, wherein: the multiple cores
comprise first and second cores; and the optical fiber further
comprises an additional core within the cladding and separated from
the first and second cores, the additional core having a
substantially circular cross-section.
4. The optical fiber of claim 3, wherein: the first core is
configured to transport a high-power laser beam; the second core is
configured to transport an illumination laser beam; and the
additional core is configured to transport an alignment laser
beam.
5. The optical fiber of claim 4, wherein: the first core is
configured to transport the high-power laser beam having an average
power of at least about 50 kW; the second core is configured to
transport the illumination laser beam having an average power of at
least about 1 kW; and the additional core is configured to
transport the alignment laser beam having an average power of
around 1 W.
6. The optical fiber of claim 1, wherein: the multiple cores
comprise first and second cores; and the first and second cores are
coaxial.
7. The optical fiber of claim 1, wherein the optical fiber is
mechanically flexible more in the fast-axis direction and less in
the slow-axis direction.
8. The optical fiber of claim 1, wherein one or more of the
multiple cores has a substantially rectangular cross-section.
9. The optical fiber of claim 1, wherein the optical fiber has a
length of at least two meters.
10. The optical fiber of claim 1, wherein the optical fiber is
configured to suppress stimulated Brillouin scattering (SBS) using
at least one of: transport of at least one laser beam having a
frequency selected so that a ratio .delta.v/.delta.v.sub.B of
signal bandwidth .delta.v to SBS gain bandwidth .delta.v.sub.B in
the optical fiber increases SBS threshold power of the optical
fiber; longitudinal variations in acoustic velocity where different
portions of the optical fiber each contain a different specific
concentration of one or more dopants; longitudinal variations in
acoustic velocity where different portions of the optical fiber
operate at different temperatures; and transverse variations in
acoustic velocity having a transverse gradient in dopant
concentrations in the optical fiber.
11. The optical fiber of claim 1, further comprising: an optical
coupler configured to launch multiple laser beams into the optical
fiber.
12. The optical fiber of claim 11, wherein the optical coupler
comprises a fused, all-glass coupler that is physically attached to
an end of the optical fiber.
13. A method comprising: generating multiple laser beams; arranging
the laser beams for entry into multiple cores of an optical fiber,
at least one of the multiple cores forming an optical waveguide and
having an elongated cross-section with a narrower dimension in a
fast-axis direction and a wider dimension in a slow-axis direction;
and transporting the laser beams using the optical fiber; wherein a
cladding surrounds the multiple cores, the cladding having a
refractive index that differs from at least one refractive index of
the multiple cores.
14. The method of claim 13, wherein: the multiple cores comprise
first and second cores; the multiple cores are stacked such that
the first core is located at least partially over the second core
in the fast-axis direction; and arranging the laser beams comprises
arranging the laser beams to enter the stacked cores.
15. The method of claim 13, wherein: the multiple cores comprise
first and second cores; the optical fiber further comprises an
additional core within the cladding and separated from the first
and second cores, the additional core having a substantially
circular cross-section; and the method further comprises generating
an additional laser beam and arranging the additional laser beam
for entry into the additional core.
16. The optical fiber of claim 15, wherein: the first core
transports a high-power laser beam; the second core transports an
illumination laser beam; and the additional core transports an
alignment laser beam.
17. The method of claim 13, wherein: the multiple cores comprise
first and second cores; the first and second cores are coaxial; and
arranging the laser beams comprises arranging the laser beams to
enter the coaxial cores.
18. The method of claim 13, further comprising: mechanically moving
the optical fiber more in the fast-axis direction and less in the
slow-axis direction.
19. The method of claim 13, further comprising suppressing
stimulated Brillouin scattering (SBS) by at least one of: selecting
a frequency of at least one laser beam to be transported over the
optical fiber so that a ratio .delta.v/.delta.v.sub.B of signal
bandwidth .delta.v to SBS gain bandwidth .delta.v.sub.B in the
optical fiber increases SBS threshold power of the optical fiber;
using longitudinal variations in acoustic velocity where different
portions of the optical fiber each contain a different specific
concentration of one or more dopants; using longitudinal variations
in acoustic velocity where different portions of the optical fiber
operate at different temperatures; and using transverse variations
in acoustic velocity having a transverse gradient in dopant
concentrations in the optical fiber.
20. The method of claim 13, further comprising: launching multiple
laser beams into the optical fiber using an optical coupler.
21. A system comprising: multiple lasers configured to generate
multiple laser beams; and an optical fiber configured to transport
the multiple beams, the optical fiber comprising: one or more cores
configured to transport the multiple beams, at least one of the one
or more cores forming an optical waveguide and having an elongated
cross-section with a narrower dimension in a fast-axis direction
and a wider dimension in a slow-axis direction; and a cladding
surrounding the one or more cores, the cladding having a refractive
index that differs from at least one refractive index of the one or
more cores.
22. The system of claim 21, wherein the one or more cores comprise
a single core configured to transport at least two of the laser
beams.
23. The system of claim 21, wherein: the one or more cores comprise
first and second cores; and the first and second cores are stacked
such that the first core is located at least partially over the
second core in the fast-axis direction.
24. The system of claim 21, wherein: the one or more cores comprise
first and second cores; and the first and second cores are
coaxial.
25. The system of claim 21, wherein: the one or more cores comprise
first and second cores; and the optical fiber further comprises an
additional core within the cladding and separated from the first
and second cores, the additional core having a substantially
circular cross-section.
26. The system of claim 21, wherein the optical fiber is configured
to suppress stimulated Brillouin scattering (SBS) using at least
one of: transport of at least one laser beam having a frequency
selected so that a ratio .delta.v/.delta.v.sub.B of signal
bandwidth .delta.v to SBS gain bandwidth .delta.v.sub.B in the
optical fiber increases SBS threshold power of the optical fiber;
longitudinal variations in acoustic velocity where different
portions of the optical fiber each contain a different specific
concentration of one or more dopants; longitudinal variations in
acoustic velocity where different portions of the optical fiber
operate at different temperatures; and transverse variations in
acoustic velocity having a transverse gradient in dopant
concentrations in the optical fiber.
27. The system of claim 21, further comprising: an optical coupler
configured to launch the laser beams into the optical fiber.
28. The system of claim 27, wherein the optical coupler comprises
one of: at least one free space lens or mirror; and a fused,
all-glass coupler that is physically attached to an end of the
optical fiber.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to the following U.S. patent
applications:
[0002] U.S. patent application Ser. No. 13/308,789 entitled "Method
and Apparatus for Fiber Delivery of High Power Laser Beams" filed
on Dec. 1, 2011; and
[0003] U.S. patent application Ser. No. 13/308,812 entitled "Method
and Apparatus for Implementing a Rectangular-Core Laser
Beam-Delivery Fiber that Provides Two Orthogonal Transverse Bending
Degrees of Freedom" filed on Dec. 1, 2011.
[0004] Both of these applications are hereby incorporated by
reference.
TECHNICAL FIELD
[0005] This disclosure is directed in general to laser systems.
More specifically, this disclosure is directed to multi-function
beam delivery fibers and a related system and method.
BACKGROUND
[0006] Optical fibers are routinely used in various fields, such as
in industrial and medical applications, to transport laser beams
from laser sources to desired locations. In these types of
applications, ordinary optical fibers with small circular cores are
suitable for transporting lower-power laser beams. These types of
optical fibers can be easily routed and rerouted in real-time to
support various functions.
[0007] In order to transport higher-power beams, larger cores are
typically needed in the optical fibers. However, conventional
optical fibers with large circular cores are often unsuitable for
use in higher-power applications, such as high-power military
applications that use laser beams of 10 kW or more. For instance,
conventional optical fibers with large circular cores are typically
highly multi-modal and/or produce excessive diffraction. As a
result, it is often difficult for these optical fibers to satisfy
both output power requirements and output beam quality
requirements. Also, conventional optical fibers with large circular
cores typically lack flexibility, which can interfere with their
use in certain applications.
[0008] Large mode area (LMA) optical fibers that can provide higher
quality beam transport have been developed. However, the power they
can transport is often limited by a number of processes, including
optical damage and stimulated Raman scattering (SRS) or stimulated
Brillouin scattering (SBS) when they exceed several meters in
length. These power and length limitations prevent LMA optical
fibers from being used in certain applications.
SUMMARY
[0009] This disclosure provides multi-function beam delivery fibers
and a related system and method.
[0010] In a first embodiment, an optical fiber includes multiple
cores and a cladding. At least one of the multiple cores forms an
optical waveguide and has an elongated cross-section with a
narrower dimension in a fast-axis direction and a wider dimension
in a slow-axis direction. The cladding surrounds the multiple cores
and has a refractive index that differs from at least one
refractive index of the multiple cores.
[0011] In a second embodiment, a method includes generating
multiple laser beams and arranging the laser beams for entry into
multiple cores of an optical fiber. At least one of the multiple
cores forms an optical waveguide and has an elongated cross-section
with a narrower dimension in a fast-axis direction and a wider
dimension in a slow-axis direction. The method further includes
transporting the laser beams using the optical fiber. A cladding
surrounds the multiple cores and has a refractive index that
differs from at least one refractive index of the multiple
cores.
[0012] In a third embodiment, a system includes multiple lasers
configured to generate multiple laser beams and an optical fiber
configured to transport the multiple beams. The optical fiber
includes one or more cores configured to transport the multiple
beams. At least one of the one or more cores forms an optical
waveguide and has an elongated cross-section with a narrower
dimension in a fast-axis direction and a wider dimension in a
slow-axis direction. The optical fiber also includes a cladding
surrounding the one or more cores and having a refractive index
that differs from at least one refractive index of the one or more
cores.
[0013] Other technical features may be readily apparent to one
skilled in the art from the following figures, descriptions, and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] For a more complete understanding of this disclosure and its
features, reference is now made to the following description, taken
in conjunction with the accompanying drawings, in which:
[0015] FIG. 1 illustrates an example laser system having a
multi-function beam delivery fiber in accordance with this
disclosure;
[0016] FIG. 2 illustrates an example beam combiner in the laser
system of FIG. 1 in accordance with this disclosure;
[0017] FIGS. 3 through 6 illustrate example multi-function beam
delivery fibers in the laser system of FIG. 1 in accordance with
this disclosure; and
[0018] FIG. 7 illustrates an example method for using a
multi-function beam delivery fiber in accordance with this
disclosure.
DETAILED DESCRIPTION
[0019] FIGS. 1 through 7, described below, and the various
embodiments used to describe the principles of the present
invention in this patent document are by way of illustration only
and should not be construed in any way to limit the scope of the
invention. Those skilled in the art will understand that the
principles of the present invention may be implemented in any type
of suitably arranged device or system.
[0020] FIG. 1 illustrates an example laser system 100 having a
multi-function beam delivery fiber in accordance with this
disclosure. As shown in FIG. 1, the system 100 includes lasers
102-106 that generate output beams 108-112, respectively. In this
example, the lasers 102-106 generate different types of beams for
different purposes or functions. For example, the laser 102
represents a high-energy laser or "HEL", which generates a
high-power output beam 108. A high-power output beam 108 is
generally any laser beam having a high power level, typically about
10 kW or greater. Example high-power lasers 102 could generate
output beams 108 of about 50 kW, 100 kW, 150 kW, 200 kW, or more.
The high-power laser 102 represents any suitable laser source
configured to generate a high-power laser output.
[0021] The laser 104 represents an illuminator laser, which
generates an illumination laser beam 110 used to illuminate or
"paint" a target. The average power of the illumination beam 110 is
typically much lower than the average power of the output beam 108.
The average power of the illumination beam 110 could, for instance,
be about 1 kW. The illuminator laser 104 represents any suitable
laser source configured to generate an illumination laser
output.
[0022] The laser 106 represents an alignment laser, which generates
an alignment laser beam 112 used to verify proper alignment of the
beams 108-112. The alignment beam 112 is typically a very low-power
beam. The average power of the alignment beam 112 could, for
instance, be about 1 W. The alignment laser 106 represents any
suitable laser source configured to generate an alignment laser
output.
[0023] The beams 108-112 are received at a beam combiner 114, which
combines or otherwise arranges the beams 108-112 in a manner
suitable for transport over an optical fiber 116. As described in
more detail below, the beam combiner 114 could arrange multiple
beams to enter into a common core of the optical fiber 116, and/or
the beam combiner 114 could arrange multiple beams to enter
different cores of the optical fiber 116. The beam combiner 114
includes any suitable structure for arranging multiple beams for
transport through a common optical fiber. One example
implementation of the beam combiner 114 is shown in FIG. 2, which
is described below.
[0024] The optical fiber 116 transports the beams 108-112 to an
intended destination. The optical fiber 116 includes any suitable
structure for carrying multiple laser beams, such as multiple
high-power laser beams or at least one high-power laser beam and at
least one other laser beam (such as an illumination beam and/or an
alignment beam). Example implementations of the optical fiber 116
are shown in FIGS. 3 through 6, which are described below. Each
implementation of the optical fiber 116 includes at least one
rectangular or other elongated (non-circular) core for transporting
at least a high-power laser beam. In particular embodiments, the
optical fiber 116 represents a passive device, meaning the optical
fiber 116 transports but does not create or amplify the beams
108-112.
[0025] In this example, one or more beam processing components 118
handle and/or modify the beams 108-112. The beams 108-112 can be
handled or modified in any suitable manner depending on the
application. In particular applications, for example, the beam
processing components 118 could include a beam director for
directing the beams in a particular direction or towards a
specified target. Any other or additional functions could be
performed by the beam processing components 118 according to
particular needs.
[0026] An application controller 120 is configured to perform
various operations to support one or more applications that involve
the system 100. For example, the application controller 120 could
control the generation of different beams 108-112 by the lasers
102-106. Note that the system 100 shown here could find use in a
wide variety of applications, including military-related
applications, and the operations performed by the application
controller 120 can vary depending on which application or
applications are supported using the system 100. The application
controller 120 includes any suitable structure for controlling the
generation of various laser beams 108-112. For instance, the
application controller 120 could include at least one processor,
microprocessor, microcontroller, digital signal processor (DSP),
application specific integrated circuit (ASIC), field programmable
gate array (FPGA), or other computing or processing device(s).
[0027] One or more additional components/systems 122 support any
other necessary or desired features to be used in particular
applications with the system 100. For example, in military
applications that use the high-power laser 102 to strike targets,
the additional components/systems 122 could include an acquisition
and tracking system used to identify and track the targets.
Location information can be used by the system 100 to direct the
output beam(s) toward(s) the targets. Any other suitable components
or systems can be used in conjunction with the lasers 102-106.
[0028] As described above, the optical fiber 116 includes at least
one rectangular or other elongated core for transporting at least
the high-power beam 108. This core represents a high aspect ratio
core (HARC) and can have any suitably high width-to-height ratio in
its cross-section, such as about 30:1 to about 100:1 or even more.
The other beams 110-112 could be transported in the same core or in
one or more different cores of the optical fiber 116, and the beams
110-112 can perform complementary functions in a fully integrated
HEL system.
[0029] One challenge in this type of system is to design a
multi-function beam-delivery fiber 116 that accommodates all of the
various types of beams while meeting rigorous boresight-alignment
tolerances among the beams. Attempting to transport multiple beams
over separate optical fibers would likely be inadequate, since the
multiple beams would have to be precisely aligned once they emerge
from the fibers. Moreover, using a single fiber with a large
circular core as the optical fiber 116 is typically undesirable
since it would likely be highly multi-modal and/or produce
excessive diffraction, and use of a large mode area (LMA) fiber is
usually limited to lower powers and fiber lengths of only a few
meters.
[0030] In FIG. 1, the beams 108-112 are transported using a single
optical fiber 116, so the beams' relative pointing directions can
be automatically maintained as they exit the fiber 116. In some
embodiments, the high-power laser 102 represents a continuous wave
laser that operates at a wavelength of about 1 .mu.m and that
generates a minimum output power of about 100 kW with a beam
quality requirement of about two or better. Also, in some
embodiments, the illuminator laser 104 represents a pulsed laser
that operates in the same wavelength range as the high-power laser
102 (although at a different specific wavelength) with a maximum
pulse length of about 100 ns, a pulse repetition frequency of about
50 kHz, and a spectral bandwidth in a range from about 10 GHz to
about 30 GHz. The illuminator laser 104 could have an average
output power around about 1 kW, but it can generate higher peak
powers since it is pulsed. Under these conditions, stimulated
Brillouin scattering represents a realistic performance challenge
to fiber delivery at lengths of ten meters or more. However, the
elongated core(s) of the optical fiber 116 can handle these beams
without suffering optical damage, significant attenuation, or
significant degradation in beam quality. This is possible even for
fiber lengths over several meters, including fiber lengths between
three and one hundred meters. In addition, in some embodiments, the
alignment laser 106 represents a laser source that outputs a
low-power beam at a visible or near-infrared wavelength, such as a
beam of about 1 W at about 800 nm. This beam does not pose
challenges from damage, SBS, or any other intensity-dependent
process, but the optical fiber 116 still propagates this beam 112
along with the other beams 108-110 while maintaining
diffraction-limited beam quality.
[0031] In particular embodiments, the optical fiber 116 satisfies
the following requirements in addition to the ability to
accommodate multiple beams. The optical fiber 116 is rectangular
with at least one high aspect ratio core. The narrow dimension of
that core is single-mode or supports a limited number of modes
(such as two to seven modes) to match the beam quality of the
high-power laser beam(s) as closely as is practical. The wide
dimension of the core can be about 1 mm (making it highly
multi-modal) to accommodate a high-power beam (such as a 100 kW
beam), although an increased width can be used for additional
margin. In order to maintain mechanical flexibility in the narrow
dimensions, the narrow fiber dimension could be no more than about
0.5 mm. Although free-space launching of the beams 108-112 into the
fiber 116 is acceptable, the fiber 116 may be compatible with some
type of optical coupler 124 for launching the beams 108-112 into
the fiber 116, as this can significantly reduce susceptibility to
misalignments. The optical coupler 124 represents any suitable
structure for launching multiple beams into an optical fiber. For
instance, the optical coupler 124 could include at least one
free-space lens or mirror. The optical coupler 124 could also
include a fused, all-glass coupler that may optionally be attached
to the end of the fiber 116 using a fusion splice, bonding, or
other coupling technique.
[0032] Various known optical fibers or waveguides violate one or
more of these requirements. For example, known optical fibers
typically include circular or annular cores, cores that are too
small for high-power applications, and/or cores that cannot be
fabricated at longer lengths (such as about ten meters or more).
The various embodiments of the optical fibers 116 described below
can satisfy the above requirements, rendering these optical fibers
116 suitable for use in various high-power applications. Moreover,
since these optical fibers 116 can exceed several meters in length,
this allows laser sources (such as lasers 102-106) to be placed in
more suitable locations on an aircraft or other structure, even if
those locations are remote from the beam processing components
118.
[0033] Although FIG. 1 illustrates one example of a laser system
100 having a multi-function beam delivery fiber 116, various
changes may be made to FIG. 1. For example, the optical fiber 116
could be used to transport any number of high-power output beams
and any number of additional beams. Also, the layout of the system
100 in FIG. 1 is for illustration only.
[0034] FIG. 2 illustrates an example beam combiner 114 in the laser
system 100 of FIG. 1 in accordance with this disclosure. As shown
in FIG. 2, the beam combiner 114 includes optics 202-206, dichroic
filters 208-210, and a mirror 212. These components operate so that
the beams 108-112 from the lasers 102-106 can be arranged and
provided to the launch end of the optical fiber 116. In this
example, the beams 108-112 have wavelengths of .lamda..sub.h,
.lamda..sub.i, and .lamda..sub.a, respectively.
[0035] Optics 202 can focus or otherwise process the high-power
beam 108, which is provided to the dichroic filter 208. Optics 204
can focus or otherwise process the illumination beam 110, which is
provided to the dichroic filter 210. Optics 206 can focus or
otherwise process the alignment beam 112, which is provided to the
mirror 212. The optics 202-206 include any suitable structures for
preparing beams for insertion into an optical fiber. For instance,
the optics 202-206 can be used to provide the desired beam shapes
and pointing directions required to launch the beams 108-112 into
the fiber 116.
[0036] The mirror 212 is highly reflective (such as about 100%) at
the wavelength .lamda..sub.a of the alignment beam 112. The
dichroic filter 210 is highly transmissive (such as about 100%) at
the wavelength .lamda..sub.a of the alignment beam 112 and highly
reflective (such as about 100%) at the wavelength .lamda..sub.i of
the illumination beam 110. The dichroic filter 208 is highly
transmissive (such as about 100%) at the wavelength .lamda..sub.h
of the high-power beam 108 and highly reflective (such as about
100%) at the wavelengths .lamda..sub.i and .lamda..sub.a of the
illumination and alignment beams 110-112. The beams 108-112 are
provided from the dichroic filter 208 to the optical fiber 116,
possibly through one or more optical couplers 124. Each dichroic
filter 208-210 includes any suitable structure for passing light at
one or more wavelengths and reflecting light at other wavelengths.
The mirror 212 includes any suitable structure that is highly
reflective at the wavelength of the alignment beam 112.
[0037] Note that while the three beams 108-112 are shown here as
entering the optical fiber 116 side-by-side, this arrangement is
for illustration only. The actual arrangement of the beams 108-112
depends on the structure of the optical fiber 116. The beams
108-112 could be collinear or separated depending on the design of
the delivery fiber.
[0038] In particular embodiments, the high-power beam 108 and the
illumination beam 110 have a wavelength separation of about 30 nm
to about 60 nm. This separation allows the filter 208 to spectrally
combine the beams 108-110. The alignment beam 112 could be at least
200 nm apart from the other beams 108-110, again providing an
adequate spectral spacing for combination with the other beams
108-110.
[0039] Although FIG. 2 illustrates one example of the beam combiner
114 in the laser system 100 of FIG. 1, various changes may be made
to FIG. 2. For example, the beams 108-112 could be combined or
otherwise arranged in any other suitable manner. Also, any number
of high-power output beams and any number of additional beams could
be combined for transport over the optical fiber 116.
[0040] FIGS. 3 through 6 illustrate example optical fibers 300-600
in the laser system 100 of FIG. 1 in accordance with this
disclosure. These optical fibers 300-600 could be used as the
optical fiber 116 in the system 100 of FIG. 1. Note, however, that
these optical fibers 300-600 could be used in any other suitable
system.
[0041] In general, the multi-function beam delivery fiber 116 can
be designed in a number of ways depending on the specific
requirements of a given application. In the following discussion,
attention is first made with respect to accommodating the
high-power beam 108 and the illumination beam 110, followed by
techniques for including the alignment beam 112 in the optical
fiber.
[0042] When a high-power beam 108 and an illumination beam 110 have
comparable beam qualities, an optical fiber 300 as shown in FIG. 3
can be used. As shown in FIG. 3, the beams 108-110 are co-aligned
as they enter the fiber 300, and the beams 108-110 remain
co-aligned when they exit the fiber 300 and exit the overall system
on a path down range. In this case, the fiber 300 can use a single
rectangular or other elongated core 302 that carries both beams
108-110. The core 302 represents an optical waveguide and is
surrounded by a cladding 304, and a coating 306 covers the cladding
304.
[0043] As shown here, the core 302 is narrower in a fast-axis
direction and wider in a slow-axis direction. The fiber 300 itself
can be flexible in the fast-axis direction and more rigid in the
slow-axis direction, and the fiber 300 can extend for lengths up to
ten meters or more. The fiber 300 could also be dynamically flexed
while in use, such as when the fiber 300 is used to provide beams
to a beam director that moves to track one or more targets. As
noted above, the narrow dimension of the core 302 can be
single-mode or support a limited number of modes (such as two to
seven modes) to match the beam qualities of the beams 108-110 as
closely as possible. The narrow dimension of the core 302 can also
be narrow enough to provide mechanical flexibility in that
direction. The wide dimension of the core 302 could be wide enough
(such as about 1 mm or more) to accommodate a single-mode
high-power beam 108.
[0044] In some implementations, the narrow or fast-axis dimension
of the core 302 and its numerical aperture (NA) can be adapted to
the desired beam quality. Based on the practice employed in fiber
delivery for high-power industrial lasers, the dimensions of the
core 302 could be designed to slightly exceed the actual beams'
dimensions, thereby providing some alignment margin. The wide or
slow-axis dimension of the core 302 can be specified based on the
total beam power to be transmitted through the core 302. The wide
dimension of the core 302 can also be specified so that both beams
108-110 can propagate along the entire length of the fiber 300
without exceeding the threshold for stimulated scattering or other
performance-limiting processes that are intensity-dependent. As
particular examples, the core 302 could have an aspect ratio
between about 30:1 to about 100:1, although other aspect ratios
could be used depending on the laser power and application. Also
note that the core 302 could have any suitable cross-sectional
area, including areas of about 10,000 .mu.m.sup.2, about 20,000
.mu.m.sup.2, about 30,000 .mu.m.sup.2, about 40,000 .mu.m.sup.2, or
even more.
[0045] The optics 202-204 in FIG. 2 can generate high aspect ratio
beam shapes for both beams 108-110 independent of their initial
beam shapes. These optics 202-204 can also include mechanisms, such
as alignment wedges, that allow the beams 108-110 to be aligned
parallel to one another. The beams 108-110 can be spatially joined
at the dichroic filter 208 and launched into the fiber 300. When
the beams 108-110 exit the fiber 300, they continue to be treated
as a single beam. For instance, they can be directed along a
free-space path that includes a deformable mirror, a fast steering
mirror, or local auto-alignment optics. This path can end as the
beams 108-110 are sent down range.
[0046] When a high-power beam 108 and an illumination beam 110 have
different beam qualities, various optical fibers could be used. One
option is to use a single core large enough to accommodate the beam
with the worse beam quality, and the beam quality of the other beam
is allowed to degrade as it propagates through a dynamically
flexing fiber. This performance degradation may be a reasonable
trade to make in favor of a simpler fiber design. In this case, the
fiber 300 shown in FIG. 3 could be used, where the dimensions of
the core 302 accommodate the beam with the worse beam quality.
[0047] A second option is shown in FIG. 4, where an optical fiber
400 has multiple rectangular or other elongated coaxial cores
402-404 with different fast-axis dimensions and/or numerical
apertures. The cores 402-404 are surrounded by a cladding 406 and a
coating 408. In this case, the wavelengths that the cores 402-404
are designed for correspond to the high-power beam 108 and the
illumination beam 110. The inner core 402 can be designed to
support the beam having the better beam quality (such as the
high-power beam 108), while the outer core 404 can be designed to
support the beam having the worse beam quality (such as the
illumination beam 110). Given the quality of state-of-the-art fiber
fabrication, the two beams 108-110 would be rigorously coaxial and
pointing in the same direction as they exit the fiber 400.
[0048] A third option is shown in FIG. 5, where an optical fiber
500 has multiple fully independent cores 502-504, one for each beam
108-110. The cores 502-504 are surrounded by a cladding 506 and a
coating 508. In this case, the cores 502-504 could have the same
vertical heights or different vertical heights, and the widths of
the cores 502-504 may or may not be equal. Also, the thicknesses of
the cladding 506 below the core 502, between the cores 502-504, and
above the core 504 may or may not be equal. For example, the
dimensional fidelity of the final core shapes might be improved if
the cladding 506 is thicker between the cores 502-504 than above
and below the cores 502-504. It may also be that the thickness of
the cladding 506 above the core 502 is less than the thickness of
the cladding 506 below the core 504 to improve its bend
performance. In particular embodiments, the total height of the
entire fiber 500 may be no more than about 0.5 mm so the fiber 500
can accommodate dynamic mechanical flexing present in some
beam-delivery applications. In the configuration shown in FIG. 5,
the cores 502-504 represent multiple stacked planar waveguides
within a common optical fiber. The cores 502-504 are said to be
stacked since one core is placed at least partially over the other
core in a direction parallel to the fast axis.
[0049] The alignment beam 112 can be transported through an optical
fiber in any suitable manner. In some embodiments, the alignment
beam 112 is single-mode at a wavelength in the visible or
near-infrared spectral region, and several options exist for a
single-mode core to transport this beam 112. For example, a
circular single-mode core could be located anywhere in the cladding
of any of the optical fibers described above. An example
implementation of this is shown in FIG. 6, which illustrates an
optical fiber 600 that is similar to the optical fiber 500. In this
example, the optical fiber 600 includes multiple elongated cores
602-604, a cladding 606, a coating 608, and a circular core 610.
The core 610 could represent a small, single-mode core suitable for
transporting the alignment beam 112. As another example, a circular
single-mode core could be coaxial with the core(s) for the other
beams 108-110. As a third example, a circular single-mode core
could be located within the core(s) for the other beams 108-110 but
displaced from the common axis of other two beams 108-110. Since
the alignment beam 112 operates at a low power and is single-mode,
the core design for this beam is readily available in the art.
[0050] Each core in the optical fibers 300-600 can be formed from
any suitable material(s), such as silica. Each cladding in the
optical fibers 300-600 can be formed from any suitable material(s),
such as at least one optical cladding material having a refractive
index that is different than the refractive index of the core(s).
Each coating in the optical fibers 300-600 can be formed from any
suitable material(s), such as a polymer. Also, each of the optical
fibers 300-600 can be fabricated in any suitable manner. For
instance, after accurate selection and purification of glass
material for a core, a fiber draw process can be used to create the
core of an optical fiber. Accurate control of the temperature
uniformity during the fiber draw process can help to increase the
quality of the fabricated core. A fiber draw process can allow
scaling of the optical fibers to lengths of ten meters or more.
[0051] As noted above, the optical fibers 300-600 can be designed
to satisfy certain stimulated Brillouin scattering (SBS)
requirements. With respect to the illumination beam 110, the peak
power of the beam 110 may be high enough to exceed the steady-state
SBS threshold of an optical fiber. For a ten-meter 10,000
.mu.m.sup.2 core silica fiber, the peak power of the beam 110 may
exceed the steady-state SBS threshold by a factor of around 200 or
more. As a result, a technique for suppressing SBS in the delivery
fiber by a factor of at least 200 could be used. In particular
embodiments, one or more of the following three techniques could be
used for SBS suppression. These three techniques can be applied
with relatively minor engineering impact on the remainder of the
system 100.
[0052] In a first technique, it is well established that the SBS
threshold can be significantly increased by using beams having
broad spectral widths. For example, the SBS gain bandwidth
.delta.v.sub.B of fused silica for a wavelength of 1 .mu.m is about
40 MHz, and the SBS threshold power as a function of the signal
bandwidth .delta.v increases approximately as the ratio
.delta.v/.delta.v.sub.B. In some embodiments, the illuminator
spectral width allows a range from about 10 GHz to about 30 GHz.
Assuming a frequency at the high end of this range is selected, the
effective SBS gain is reduced by a factor of 750 (30 GHz/40 MHz),
which takes the system below the SBS threshold. Even though the
peak power of the illumination beam 110 exceeds the average power
of the high-power beam 108, selecting a judicious set of operating
parameters for the illumination beam 110 can avoid SBS in the
delivery fiber.
[0053] In a second technique, longitudinal variations in acoustic
velocity are used. This technique recognizes that acoustic velocity
and SBS frequency shift can be changed by incorporating various
dopant ions into the fiber material. If a dopant-induced frequency
shift exceeds the SBS gain bandwidth, one portion of the fiber is
effectively decoupled from other portions of the fiber, thereby
reducing the effective SBS interaction length and increasing the
SBS threshold power. This technique can be exploited if the
beam-delivery fiber is formed by splicing together two or more
fiber sections. Each fiber section can have a different acoustic
velocity associated with its specific concentration of one or more
dopants, such as germanium or fluorine. This technique also
recognizes that acoustic velocity and SBS frequency shift can be
changed by operating different portions of the optical fiber at
different temperatures. Even if this technique by itself provides
inadequate SBS suppression at power levels such as 100 kW or more,
it can certainly be used in other applications involving lower peak
powers or in conjunction with one or more other SBS-suppression
techniques to provide additional margin.
[0054] In a third technique, transverse variations in acoustic
velocity are used. This technique also exploits the
dopant-dependence of acoustic velocity, but it achieves SBS
suppression using a prescribed transverse gradient in the dopant
concentrations. In this approach, the following three things can be
recognized. One, the acoustic wave involved in the SBS process has
a wavelength that is half of the optical wavelength, so the
acoustic wave can also be guided in a fiber core. Two, optical
guiding in a fiber depends on the transverse profile of the
refractive index, which is the factor by which the speed of light
is reduced in an optical material relative to a vacuum. In an
analogous manner, the degree of acoustic guiding depends on the
"acoustic index" of the core material, which is associated with the
relative value of the acoustic velocity in the material as a
function of the dopant concentrations. Three, SBS depends on the
spatial overlap of the optical and acoustic waves within the fiber
material, so reducing this spatial overlap can raise the SBS
threshold. The details of how a fiber core can be designed to
exploit this type of SBS suppression are well known in the art. In
general, the core design concentrates the optical wave along the
fiber axis, while the acoustic wave tends to be concentrated in an
annular region radially displaced from the core axis. This spatial
separation of the optical and acoustic wave amplitudes reduces the
effective SBS gain and raises the SBS threshold.
[0055] Stimulated Raman scattering (SRS) may also be a
consideration in the design of the optical fibers 300-600. The SRS
gain in fused silica is about 300 times weaker than the SBS gain.
Therefore, according to the example above where the intensity of
the illumination beam 110 exceeds the SBS threshold by a factor of
about 200, the illuminator power would be about 2/3 of the SRS
threshold level, so no SRS suppression is needed. If higher
illuminator powers are used, any suitable suppression technique
could be used to reduce SRS. For instance, the cross-sectional area
of the fiber core transporting the illumination beam 110 could be
increased, thereby maintaining or even reducing the signal
intensity for the same signal power and avoiding any SRS.
[0056] Although FIGS. 3 through 6 illustrate examples of optical
fibers 300-600 in the laser system 100 of FIG. 1, various changes
may be made to FIGS. 3 through 6. For example, each fiber core
could be placed at any suitable location within the cladding of an
optical fiber and need not be centered. Also, the size and shape of
each core shown in FIGS. 3 through 6 are for illustration only. In
addition, while the optical fibers 300-600 are described as
transporting a high-power beam and an illumination beam (and
possibly an alignment beam), the optical fibers 300-600 could be
used to transport multiple beams of any suitable type(s), and more
than two or three beams could be transported.
[0057] FIG. 7 illustrates an example method 700 for using a
multi-function beam delivery fiber in accordance with this
disclosure. As shown in FIG. 7, multiple laser beams are generated
at step 702. This could include, for example, different lasers
102-106 generating a high-power beam 108, an illumination beam 110,
and an alignment beam 112. The multiple beams are arranged to enter
a single optical fiber at step 704. This could include, for
example, the beam combiner 114 arranging the beams 108-112 into
suitable positions for entry into the optical fiber 114. The exact
arrangement of the beams 108-112 can vary depending on the number
and position(s) of the core(s) in the optical fiber 116. The beams
108-112 could be collinear or separated.
[0058] The beams are provided to one or more cores of the optical
fiber at step 706. This could include, for example, using one or
more optical couplers 124 to couple the beams 108-112 into one or
more cores of the optical fiber 116. Free space coupling could also
be used. As noted above, the beams 108-112 could be sent through a
single core in the optical fiber (such as is shown in FIG. 3) or
through multiple cores in the optical fiber (such as is shown in
FIGS. 4 through 6). The beams are transported using the optical
fiber at step 708 and delivered to an intended destination using
the optical fiber at step 710. Any suitable components could use
the beams at the destination, such as when a beam director aims the
beams at one or more targets.
[0059] Although FIG. 7 illustrates one example of a method 700 for
using a multi-function beam delivery fiber, various changes may be
made to FIG. 7. For example, while shown as a series of steps,
various steps in FIG. 7 can overlap, occur in parallel, or occur
any number of times. Also, the method 700 could involve the
generation and transport of any plural number and type(s) of
beams.
[0060] It may be advantageous to set forth definitions of other
certain words and phrases used throughout this patent document. The
terms "include" and "comprise," as well as derivatives thereof,
mean inclusion without limitation. The term "or" is inclusive,
meaning and/or. The phrase "associated with," as well as
derivatives thereof, may mean to include, be included within,
interconnect with, contain, be contained within, connect to or
with, couple to or with, be communicable with, cooperate with,
interleave, juxtapose, be proximate to, be bound to or with, have,
have a property of, have a relationship to or with, or the
like.
[0061] While this disclosure has described certain embodiments and
generally associated methods, alterations and permutations of these
embodiments and methods will be apparent to those skilled in the
art. Accordingly, the above description of example embodiments does
not define or constrain this disclosure. Other changes,
substitutions, and alterations are also possible without departing
from the spirit and scope of this disclosure, as defined by the
following claims.
* * * * *